Free PDF Download of CBSE Physics Multiple Choice Questions for Class 12 with Answers Chapter 7 Alternating Current. Physics MCQs for Class 12 Chapter Wise with Answers PDF Download was Prepared Based on Latest Exam Pattern. Students can solve NCERT Class 12 Physics Alternating Current MCQs Pdf with Answers to know their preparation level.
Alternating Current Class 12 Physics MCQs Pdf
1. The phase difference between the alternating current and emf is TE/2. Which ofthe following cannot be the constituent of the circuit?
(a) C alone
(b) L alone
(c) L and C
(d) R and L
Answer/Explanation
Answer: c
Explaination:
(c) When both L and C components are there then phase difference could be zero, less than or greater than \(\frac{\pi}{2}\).
2. In an LCR-series ac circuit, the voltage across each of the component L, C and R is 50 V. The voltage across the LC-combination will be
(a) 50 V
(b) 50√2 V
(c) 100 V
(d) zero
Answer/Explanation
Answer: d
Explaination:
(d) The voltage across L and C are out of phase. Hence the voltage across the LC combination is zero.
Physics MCQs with Answers for Class 12 Question 3. An ac circuit has a resistance of 12 ohm and an impedance of 15 ohm. The power factor of the circuit will be
(a) 0.8
(b) 0.4
(c) 0.125
(d) 1.25
Answer/Explanation
Answer: a
Explaination:
(a) Power factor, cos Φ = \(\frac{R}{Z}\) = \(\frac{12}{15}\) = 0.8
4. In an ac circuit the voltage applied is s = s0 sin (tit. The resulting current in the circuit is I = 70 sin (cot – nil). The power consumption in the circuit is given by
Answer/Explanation
Answer: c
Explaination:
(c) Here, Φ = \(\frac{\pi}{2}\),
SO pav = εrms Irms COS Φ = 0
Physics MCQs for Class 12 Chapter wise with Answers Pdf Question 5. In an LCR circuit, capacitance is charged from C to 2C. For resonant frequency to remain unchanged, the inductance should be changed from L to
(a) 4 L
(b) 2 L
(c) L/2
(d) L/4
Answer/Explanation
Answer: c
Explaination:
(c) Since, Resonance frequency,
\(v_{r}=\frac{1}{2 \pi \sqrt{L C}}\)
when C is changed 2C, L should be change to L/2.
6. The core of any transformer is laminated so as to
(a) reduce the energy loss due to eddy currents.
(b) make it light weight.
(c) make it robust and strong.
(d) increase the secondary voltage.
Answer
Answer: a
MCQ Questions for Class 12 Physics with Answers Pdf Question 7. In an a.c. generator, a coil with N turns, all of the same area A and total resistance R, rotates with frequency ω in a magnetic field B the maximum value of emf generated in the coil is
(a) NABR
(b) NABω
(c) NABRω
(d) NAB
Answer/Explanation
Answer: b
Explaination:
(b) emf induced, ε = NABGω sin ωt,
ε0 = NBAω
8. If coil is open, then L and R becomes
(a) infinity, zero
(b) zero, infinity
(c) infinity, infinity.
(d) zero, zero
Answer/Explanation
Answer: b
Explaination:
(b) No current flows through an open circuit so magnetic flux is zero and hence, L = 0.
Also as I = 0, so R = ∞.
Physics Multiple Choice Questions with Answers for Class 12 Pdf Question 9. In a series, LCR-circuit, resonant frequency depends on
Answer/Explanation
Answer: c
Explaination: (c) Resonance frequency, vr = \(\frac{1}{\sqrt{LC}}\)
10. Which quantity is increased in a step-down transformer?
(a) Current
(b) Voltage
(c) Power
(d) Frequency
Answer/Explanation
Answer: a
Explaination: (a) In a step-down transformer current increases and voltage decreases.
11. If an AC voltage is applied to an ICi? circuit, which of the following is true?
(a) I and V are out of phase with each other inR.
(b) I and V are in phase in L while in C, they are out of phase.
(c) I and V are out of phase in both C and L.
(d) I and V are out of phase in L and in phase in C.
Answer
Answer: c
12. The peak value of ac voltage on a 220 V mains is
(a) 200√2 V
(b) 230√2 V
(c) 220√2 V
(d) 240√2 V
Answer/Explanation
Answer: c
Explaination:
(c) Peak value of ac voltage V0 = √2 Vrms.
= 220√2 V.
13. Series ac circuit has inductance L, resistance R and angular frequency to, the quality factor Q is
Answer/Explanation
Answer: b
Explaination:
(b) Quality factor is measure of sharpness of resonance, is equal to \(\frac{\omega L}{R}\).
Physics MCQ Questions for Class 12 Pdf Question 14. A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 A, the efficiency of the transformer is approximately
(a) 30%
(b) 50%
(c) 90%
(d) 10%
Answer/Explanation
Answer: c
Explaination:
(c) Since efficiency of a transformer,
15. Choose the correct statement.
(a) A capacitor can conduct a dc circuit but not an inductor.
(b) In a dc circuit the inductor can conduct but not a capacitor.
(c) In dc circuit both the inductor and capacitor cannot conduct.
(d) The inductor has infinite resistance in a dc circuit.
Answer/Explanation
Answer: b
Explaination:
(b) In dc circuit, an inductor offers zero resistance to dc, while a capacitor offers infinite resistance.
16. What is the value of inductance L for which the current is maximum in a series LCR- circuit with C = 10 μF and ω = 1000 s-1?
(a) 100 mH
(b) 1 mH
(c) 10 mH
(d) cannot be calculated unless R is known
Answer/Explanation
Answer: a
Explaination:
(a) Current is maximum at resonance, when
17. A coil of self-inductance L is connected in series with a bulb B and an ac source. Brightness of the bulb decreases when
(a) frequency of the ac source is decreased.
(b) number of turns in the coil is reduced.
(c) a capacitance of reactance Xc = XL in included.
(d) an iron rod is inserted in the coil.
Answer/Explanation
Answer: d
Explaination:
(d) Brightness of the bulb decreases when inductive reactance increases, i.e. when iron rod is inserted in the coil, its inductance L increases, hence current decreases.
MCQs Of Physics Class 12 Question 18. The reactance of a capacitor C is X. If both the frequency and capacitance be doubled, then new reactance will be
(a) X
(b) 2X
(c) 4X
(d) \(\frac{X}{4}\)
Answer/Explanation
Answer:
Explaination:
when f and C both are doubled.
19. A transformer works on the principle of
(a) converter.
(b) inverter.
(c) mutual inductance.
(d) self-inductance.
Answer/Explanation
Answer: c
Explaination: (c) Transformer works on the principle of mutual inductance.
20. The line that draws power supply to your house from street has
(a) zero average current.
(b) 220 V average voltage.
(c) voltage and current out of phase by 90°.
(d) voltage and current are in phase.
Answer/Explanation
Answer: a
Explaination: (a) zero average current.
21. Alternating current cannot be measured by dc ammeter, because
(a) ac cannot pass through ac ammeter.
(b) ac charges direction.
(c) average value of current of complete cycle is zero.
(d) ac ammeter will get damaged.
Answer/Explanation
Answer: a
Explaination: (a) ac cannot pass through ac ammeter.
22. Average power generated in an inductor connected to an a.c. source is
(a) \(\frac{1}{2}\)LI²
(b) LI²
(c) zero
(d) none of these
Answer/Explanation
Answer: c
Explaination:
(c) Average power dissipated in an inductor connected to an ac source is zero,
Pav = V0I0 cos = 0(Phase angle is \(\frac{\pi}{2}\))
23. The power factor varies between
(a) 2 and 2.5
(b) 3.5 to 5
(c) 0 to 1
(d) 1 to 2
Answer/Explanation
Answer: c
Explaination: (c) Power factor cos Φ in an ac circuit varies between 0 and 1.
24. Reciprocal of impedance is
(a) susceptance
(b) conductance
(c) admittance
(d) transconductance
Answer/Explanation
Answer: c
Explaination: (c) admittanc
25. Alternating voltage (V) is represented by the equation
(a) V(t) = Vm eωt
(b) V(t) = Vm sin ωt
(c) V(t) = Vm cot ωt
(d) V(t) = Vm tan ωt
Answer
Answer: b
26. The rms value of potential difference V shown in the figure is
Answer
Answer: c
27. The phase relationship between current and voltage in a pure resistive circuit is best represented by
Answer
Answer: b
28. In the case of an inductor
(a) voltage lags the current by \(\frac{π}{2}\)
(b) voltage leads the current by \(\frac{π}{2}\)
(c) voltage leads the current by \(\frac{π}{3}\)
(d) voltage leads the current by \(\frac{π}{4}\)
Answer
Answer: b
29. Which of the following graphs represents the correct variation of inductive reactance XL with frequency u?
Answer
Answer: b
30. In a pure capacitive circuit if the frequency of ac source is doubled, then its capacitive reactance will be
(a) remains same
(b) doubled
(c) halved
(d) zero
Answer
Answer: c
31. Which of the following graphs represents the correct variation of capacitive reactance Xc with frequency v u?
Answer
Answer: c
32. In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?
(a) Only resistor
(b) Resistor and inductor
(c) Resistor and capacitor
(d) Only inductor
Answer
Answer: c
33. In which of the following circuits the maximum power dissipation is observed?
(a) Pure capacitive circuit
(b) Pure inductive circuit
(c) Pure resistive circuit
(d) None of these
Answer
Answer: c
34. In series LCR circuit, the phase angle between supply voltage and current is
Answer
Answer: a
35. In a series LCR circuit the voltage across an inductor, capacitor and resistor are 20 V, 20 V and 40 V respectively. The phase difference between the applied voltage and the current in the circuit is
(a) 30°
(b) 45°
(c) 60°
(d) 0°
Answer
Answer: d
36. At resonance frequency the impedance in series LCR circuit is
(a) maximum
(b) minimum
(c) zero
(d) infinity
Answer
Answer: b
37. At resonant frequency the current amplitude in series LCR circuit is
(a) maximum
(b) minimum
(c) zero
(d) infinity
Answer
Answer: a
38. Quality factor and power factor both have the dimensions of
(a) time
(b) frequency
(c) work
(d) angle
Answer
Answer: d
39. The natural frequency (ca0) of oscillations in LC circuit is given by
Answer
Answer: c
40. A transformer works on the principle of
(a) self induction
(b) electrical inertia
(c) mutual induction
(d) magnetic effect of the electrical current
Answer
Answer: c
41. For an ideal-step-down transformer, the quantity which is constant for both the coils is
(a) current in the coils
(b) voltage across the coils
(c) resistance of coils
(d) power in the coils
Answer
Answer: d
42. If the rms current in a 50 Hz ac circuit is 5 A, the value of the current 1/300 seconds after its value becomes zero is [NCERT Exemplar]
(a) 5√2A
(b) \(5 \sqrt{3 / 2}\)A
(c) 5/6 A
(d) 5√2A
Answer
Answer: b
43. An alternating current generator has an internal resistance Rg and an internal reactance Xg. It is used to supply power to a passive load consisting of a resistance Rg and a reactance XL For maximum power to be delivered from the generator to the load, the value of XL is equal to [NCERT Exemplar]
(a) zero.
(b) Xg.
(c) -Xg.
(d) Rg.
Answer
Answer: c
44. When a voltage measuring device is connected to AC mains, the meter shows the steady input voltage of 220 V. This means [NCERT Exemplar]
(a) input voltage cannot be AC voltage, but a DC voltage.
(b) maximum input voltage is 220V.
(c) the meter reads not v but < v² > and is . calibrated to read √< v² > .
(d) the pointer of the meter is stuck by some mechanical defect.
Answer
Answer: c
45. To reduce the resonant frequency in an LCR series circuit with a generator [NCERT Exemplar]
(a) the generator frequency should be reduced.
(b) another capacitor should be added in parallel to the first.
(c) the iron core of the inductor should be removed.
(d) dielectric in the capacitor should be removed.
Answer
Answer: b
46. Which of the following combinations should be selected for better tuning of an LCR circuit used for communication? [NCERT Exemplar]
(a) R = 20, L= 1.5 H, C = 35 μF.
(b) R = 25 , L = 2.5 H, C = 45 μF.
(c) R = 15 , L = 3.5 H, C = 30 μF.
(d) R = 25 , L = 1.5 H, C = 45 μF.
Answer
Answer: c
47. An inductor of reactance 1 Ω and a resistor of 2 Ω are connected in series to the terminals of a 6 V (rms) a.c. source. The power dissipated in the circuit is [NCERT Exemplar]
(a) 8 W.
(b) 12 W.
(c) 14.4 W.
(d) 18 W.
Answer
Answer: c
48. The output of a step-down transformer is measured to be 24 V when connected to a 12 watt light bulb. The value of the peak current is [NCERT Exemplar]
(a) 1/√2 A.
(b) √2A.
(c) 2 A.
(d) 2√2 A.
Answer
Answer: a
49. As the frequency of an ac circuit increases, the current first increases and then decreases. What combination of circuit elements is most likely to comprise the circuit? [NCERT Exemplar]
(a) Inductor and capacitor.
(b) Resistor and inductor.
(c) Resistor and capacitor.
(d) Inductor only.
Answer
Answer: a
50. In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?
(a) Only resistor.
(b) Resistor and an inductor.
(c) Resistor and a capacitor.
(d) Only an inductor.
Answer
Answer: c
51. Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is not correct?
(a) For a given power level, there is a lower current.
(b) Lower current implies less power loss.
(c) Transmission lines can be made thinner.
(d) It is easy to reduce the voltage at the receiving end using step-down transformers.
Answer
Answer: c
52. For an LCR circuit, the power transferred from the driving source to the driven oscillator is P = I²Z cos Φ. Which of the following is incorrect?
(a) Here, the power factor cos Φ ≥ 0, P ≥ 0.
(b) The driving force can give no energy to the oscillator (P = 0) in some cases.
(c) The driving force cannot syphon out (P < 0) the energy out of oscillator.
(d) The driving force can take away energy out of the oscillator.
Answer
Answer: d
53. When an AC voltage of 220 V is applied to the capacitor C
(a) the maximum voltage between plates is 220 V.
(b) the current is in phase with the applied voltage.
(c) the charge on the plates is not in phase with the applied voltage.
(d) power delivered to the capacitor is zero.
Answer
Answer: d
54. Average value of ac over a complete cycle _________ .
Answer/Explanation
Answer:
Explaination: zero
55. Inductive reactance increases with _________ in frequency of ac.
Answer/Explanation
Answer:
Explaination: increase
56. In ac generator _________ is converted into electrical energy of alternating form.
Answer/Explanation
Answer:
Explaination: mechanical energy
57. Electric current that varies in magnitude dontinuously and reverses its direction periodically is known as _________ .
Answer/Explanation
Answer:
Explaination: alternating current.
58. Current which flows with a constant magnitude in same fixed direction is known as _________ .
Answer/Explanation
Answer:
Explaination: direct current.
59. Average or mean value of ac over a half cycle is _________ .
Answer/Explanation
Answer:
Explaination: 0.637 I0
60. Rms or virtual value of alternating current is _________ .
Answer/Explanation
Answer:
Explaination: 0.707 I0
61. Current in the inductive a.c. circuit _________ the voltage.
Answer/Explanation
Answer:
Explaination: lag behind
62. Capacitive reactance increases with _________ in frequency of ac.
Answer/Explanation
Answer:
Explaination: increase
63. Capacitive reactance Xc increases with _________ in frequency of ac and current in the capacitive ac circuit _________ the ac voltage.
Answer/Explanation
Answer:
Explaination: decrease, leading ahead
64. Can ever the rms value be equal to the peak value of an ac?
Answer/Explanation
Answer:
Explaination: Yes, when the ac is a square wave.
65. Which is more dangerous 220 ac or 220 dc and why?
Answer/Explanation
Answer:
Explaination:
∴ ac is more dangerous than dc because its peak value is very high and also ac is in nature.
66. The peak value of emf in ac is E0. Write its
(i) rms, and (ii) average value over a complete cycle. [Foreign 2011]
Answer/Explanation
Answer:
Explaination: (i) Erms = \(\frac{E_{0}}{\sqrt{2}}\) (ii) Zero.
67. The current flowing through a pure inductor of inductance 4mH is i= 12 cos 300 t ampere. What is (i) rms, and (ii) average value of the current for a complete cycle? [Foreign 2011]
Answer/Explanation
Answer:
Explaination: (i) Irms = \(\frac{12}{\sqrt{2}}\) (ii) Iav = zero
68. Calculate the rms value of the alternating current shown in the figure. [HOTS]
Answer/Explanation
Answer:
Explaination:
The rms value of the alternating current shown in the figure is 2 A.
69. When an alternating current is passed through a moving coil galvanometer, it shows no deflection. Why?
Answer/Explanation
Answer:
Explaination: A moving coil galvanometer measures an average value of current, which is zero for ac. Hence, no deflection is shown by galvanometer
70. Which of the following curves may represent the reactance of a series LC combination? [DoE]
Answer/Explanation
Answer:
Explaination:
(b) AS XC-XL = \(\frac{1}{2 \pi v C}\) – 2πvL
71. In a series LCR circuit, the voltages across an inductor, a capacitor and a resistor are 30 V, 30V and 60V respectively. What is the phase difference between the applied voltage and the current in the circuit?
Answer/Explanation
Answer:
Explaination: Zero. As VL = VC, circuit is resistive in nature.
72. Can a capacitor of suitable capacitance replace a inductor coil in an AC circuit?
Answer/Explanation
Answer:
Explaination: Yes, because average power consumed in both is least while controlling an AC.
73. At an airport, a person is made to walk through the doorway of a metal detector, for security reasons. Is she/he is carrying anything made of metal, the metal detector emits a sound. On what principle does this detector work? [NCERT Exempler]
Answer/Explanation
Answer:
Explaination: The metal detector works on the principle of resonances in ac circuits.
74. A capacitor blocks dc and allows ac. Why?
Answer/Explanation
Answer:
Explaination:
The capacitive reactance is XC =\(\frac{1}{2 \pi v C}\).
For dc, v = 0 ⇒ XC = ∞
i.e. a capacitor offers infinite resistance to dc and hence blocks it.
For ac, v ≠ 0, ⇒ XC ≠ ∞ , but has some finite value.
Therefore, an ac can pass through the capacitor.
75. A bulb and a capacitor are connected in series to an ac source of variable frequency. How will the brightness of the bulb change on increasing the frequency of the ac source?
Answer/Explanation
Answer:
Explaination:
The brightness will increase (∵ XC ∝ \(\frac{1}{v}\)), and heat produced H ∝ I², where
76. Is there are any device by which direct current can be controlled without any loss of energy?
Answer/Explanation
Answer:
Explaination:
By the definition, value of the alternating voltage is equal to 110V.
77. A 110 V dc heater is used on an ac source, such that the heat produced is the same. What would be the value of the alternating voltage?
Answer/Explanation
Answer:
Explaination: cos Φ = 1.
78. The power factor of an ac circuit is 0.5. What is the phase difference between voltage and current in this circuit? [Foreign 2015]
Answer/Explanation
Answer:
Explaination:
∵ Power factor, cos Φ=0.5
∴ Φ = 60° or π/3
So, the phase difference between voltage and current is \(\frac{\pi}{3}\).
79. Why does a low power factor imply large power loss in transmission for circuits used for transporting electric power? [NCERT Example]
Answer/Explanation
Answer:
Explaination:
We know that P = TV cos Φ, where cos Φ is the power factor. To supply a given power, at a given voltage, if cos Φ is small, we have to increase the current accordingly. But this will lead to large power loss (I²R) in transmission.
80. The instantaneous current and voltage of an ac circuitaregivenby I = 10sin314tA and V=50sin (314 t + \(\frac{\pi}{2}\)) V. What is the power of dissipation m the circuit?
Answer/Explanation
Answer:
Explaination:
Phase difference between the current and voltage is π/2. So, the power dissipation Pav = Prms cos Φ is zero.
81. Why is the use of ac voltage preferred over dc voltage? [AI2014]
Answer/Explanation
Answer:
Explaination:
An ac voltage can be stepped up or down using a transformer, but not the dc voltage.
82. Does a step down transformer violate the principle of conservation of energy?
Answer/Explanation
Answer:
Explaination:
No. In a transformer, if a voltage is increased, the current is decreased in the same ratio and the product VI (power) remains the same.
83. Can we measure 110 V, 50 Hz a.c. using moving coil galvanometers? How can we measure it.
Answer/Explanation
Answer:
Explaination:
No.
Reason: As average value of a.c. voltage over one complete cycle is zero.
We can measure the rms value of a.c. voltage using hot wire meters.
84. What is an acceptor circuit and where it is used?
Answer/Explanation
Answer:
Explaination: Series LCR circuit is called acceptor circuit radio or TV sets.
85. Why do we prefer carbon brushes than copper in an ac generator? [DoE]
Answer/Explanation
Answer:
Explaination: Corrosion free and small expansion on heating maintains proper contact.
86. In a series LCR circuit, XL, Xc and R are the inductive reactance, capacitive reactance and resistance respectively at a certain frequency f If the frequency of ac is doubled, what will be the values of reactances and resistance of the circuit? [DoE]
Answer/Explanation
Answer:
Explaination:
Resistance R remains unchanged; XL will be doubled and XC will be halved.
87. In a series L-R circuit, XL = R and power factor of the circuit is P1 When a capacitor with capacitance C such that XL = Xc is put in series, the power factor becomes P2 Find P1/P2
Answer/Explanation
Answer:
Explaination: 1 : √2
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